Growing a surface-parallel shear crack by a gradual pore pressure increase: At what point is it unstable?
Abstract
Increases in pore pressure are often cited as potential mechanisms for landslide initiation, both subaerial and submarine. Such mechanisms are particularly appealing to account for failures in the submarine environment, where shallow slope angles seem to preclude downslope movement. Considering the submarine environment, the rapid deposition of sediment can create a regime of overpressure driving the upward flow of pore fluid to the seafloor. For such environments, we propose a simple mechanism whereby the shear of loosely consolidated sediment and its subsequent compaction (and consequent reduction in permeability) result in gradually increasing pore pressures in both extent and magnitude. Here, we examine how the quasi-static increase of a spatially peaked pore-pressure profile (such as that invoked above) enlarges a shear crack within a linear-elastic body under plane-strain conditions. Specifically, when slope movement occurs, if the depth range that accommodates much of the downslope shear is small in comparison to other lengths, that region of shear may be well approximated by a slipping surface. Here, the frictional strength along the slip surface is determined by the product of the local effective normal stress and a slip-weakening friction coefficient. Ultimately, we are interested in determining the critical length of the rupture at which point its growth becomes unstable—i.e., when the sliding region extends without a further increase in the imposed pore pressure. We find that broadly peaked pore pressure profiles result in the shortest critical lengths and require the least pore pressure increase, beyond that for local slip inception, to reach the unstable state. For more realistic landslide modeling we consider a crack paralleling a nearby free surface, and report how the proximity of such surface alters results for nucleation in effectively unbounded systems.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMNH41C1272V
- Keywords:
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- 1810 HYDROLOGY / Debris flow and landslides;
- 1822 HYDROLOGY / Geomechanics;
- 3070 MARINE GEOLOGY AND GEOPHYSICS / Submarine landslides